Nonaqueous electrolyte secondary battery separator, nonaqueous electrolyte secondary batter member, and nonaqueous electrolyte secondary battery

Abstract

As a nonaqueous electrolyte secondary battery separator which has improved mechanical strength and which retains flexibility, provided is a nonaqueous electrolyte secondary battery separator including: a mixed layer which contains a heat-resistant resin and a porous base material that includes a porous film containing a polyolefin-based resin as a main component; and a heat-resistant layer which contains the heat-resistant resin and which is in contact with the mixed layer. When an SEM image which includes a cross section of the nonaqueous electrolyte secondary battery separator in a thickness direction is analyzed, luminance distribution of the mixed layer in the thickness direction satisfies specific conditions.

1 . A nonaqueous electrolyte secondary battery separator comprising:
 a mixed layer which contains a heat-resistant resin and a porous base material that includes a porous film containing a polyolefin-based resin as a main component; and   a heat-resistant layer which contains the heat-resistant resin and which is in contact with the mixed layer,   when an SEM image which includes a cross section of the nonaqueous electrolyte secondary battery separator in a thickness direction is analyzed, luminance distribution of the mixed layer in the thickness direction satisfying the following conditions:
 condition 1 a luminance X 1  is not less than 20%; and 
 condition 2 a luminance X 2  is not less than 9%, where
 a luminance at a point at a depth of 10% of a thickness of the porous base material from an interface between the mixed layer and the heat-resistant layer is X 1 %, 
 a luminance at a point at a depth of 30% of the thickness of the porous base material from the interface between the mixed layer and the heat-resistant layer is X 2 %, and 
 an average of luminances of the entire heat-resistant layer is 100%. 
 
   
     
     
         2 . The nonaqueous electrolyte secondary battery separator as set forth in  claim 1 , wherein the heat-resistant layer further contains a filler. 
     
     
         3 . The nonaqueous electrolyte secondary battery separator as set forth in  claim 2 , wherein an amount of the filler contained in the heat-resistant layer is not less than 20% by weight and not more than 90% by weight, relative to a total weight of the heat-resistant layer. 
     
     
         4 . The nonaqueous electrolyte secondary battery separator as set forth in  claim 1 , wherein a value, represented by the following expression (1), of the heat-resistant layer is not less than 5%:
               a           luminance X     3       %          - a luminance X     4       %             ­­­expression (1)                 where the luminance X 3  indicates an average value of luminances of a part from the interface between the heat-resistant layer and the mixed layer to a depth of 20% of a thickness of the heat-resistant layer from the interface,   the luminance X 4  indicates an average value of luminances of a part from an outermost surface of the heat-resistant layer to a depth of 20% of the thickness of the heat-resistant layer from the outermost surface, and   an average value of luminances of the entire heat-resistant layer is 100%.   
     
     
         5 . The nonaqueous electrolyte secondary battery separator as set forth in  claim 1 , wherein an air permeability of the nonaqueous electrolyte secondary battery separator is not more than 500 sec/100 mL. 
     
     
         6 . The nonaqueous electrolyte secondary battery separator as set forth in  claim 1 , wherein the heat-resistant resin is an aramid resin. 
     
     
         7 . A nonaqueous electrolyte secondary battery member comprising:
 a positive electrode;   a nonaqueous electrolyte secondary battery separator recited in  claim 1 ; and   a negative electrode,   the positive electrode, the nonaqueous electrolyte secondary battery separator, and the negative electrode being disposed in this order.   
     
     
         8 . A nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte secondary battery separator recited in  claim 1 . 
     
     
         9 . A nonaqueous electrolyte secondary battery comprising a nonaqueous electrolyte secondary battery member recited in  claim 7 .